The present invention relates to filter circuits suitable for use in communication systems having asymmetric communication channels such as ADSL communication. The invention also relates to front ends having the filter circuits and communication devices having the same.
In communication systems having asymmetric communication channels, such as ADSL communication, the communication channels are asymmetric such that the downstream data rate is fast whereas the upstream data rate is relatively slow. Asymmetric communication systems have received considerable attention as they can achieve high-speed communication at low cost. Cost reduction of the equipment for realizing such communication systems has also become an issue. In addition, in order to reduce the size of the front end, which is a transmitting and receiving section in such a communication system, it has been required to reduce the circuit area of the front end.
FIG. 6 is a block diagram showing the configuration of a conventional front end in an asymmetric communication system, which serves as a transmitting and receiving section. The front end is provided with a signal receiving terminal 51, and communication signals received at the signal receiving terminal 51 are input to a receiver 52.
The front end is further provided with a first to a third filter circuits 53, 54, and 55. The first filter circuit 53 and the third filter circuit 55 have a high cutoff frequency, whereas the second filter circuit 54 has a low cutoff frequency.
The output from the receiver 52 is filtered by either one of the first filter circuit 53 or the third filter circuit 55, is thereafter converted in to a digital signal by a receiving-side A-D converter 58, and is then output from a digital signal output terminal 59 to a digital processing section, which is not shown in the drawing.
The digital signal processed in the digital processing section is input from a digital signal input terminal 61, is thereafter converted into an analog signal by a transmitting-side D-A converter 62, and is then filtered by either one of the second filter circuit 54 or the third filter circuit 55. The filtered analog signal is transmitted from a signal-transmitting terminal 65 by a driver 64.
The output from the receiver 52 is supplied to the first filter circuit 53 directly, and is also supplied to the third filter 55 via an input switching circuit 57. The input switching circuit 57 is also supplied with the output from the transmitting-side D-A converter 62.
The output from the first filter circuit 53 is supplied to the A-D converter 58 via a first output switching circuit 56. The first output switching circuit 56 is also supplied with the output from the third filter circuit 55.
The output from the transmitting-side D-A converter 62 is supplied to the third filter circuit 55 via the input switching circuit 57, as described above, and is also supplied to the second filter circuit 54 directly. The output from the second filter circuit 54 is supplied to the driver 64 via a second output switching circuit 63. The second output switching circuit 63 is also supplied with the output from the third filter circuit 55.
In the front end thus configured, according to switching operations of the input switching circuit 57 and the first output switching circuit 56, the output from the receiver 52 is filtered by either one of the first filter circuit 53 or the third filter circuit 55, which is thereafter supplied to the A-D converter 58.
The output from the transmitting-side D-A converter 62 is filtered by either one of the second filter circuit 54 or the third filter circuit 55 according to switching operations of the input switching circuit 57 and the second output switching circuit 63, which is thereafter supplied to the driver 64.
The third filter circuit 55 is selected by the input switching circuit 57 so that it is used for a filtering process for only one of the received signals or the transmitted signals.
FIG. 7 shows a block diagram schematically illustrating the configuration of a filter circuit used for the first to third filter circuits 53 to 55. Normally, a filter circuit 70 is constructed such that a resistor block 72 that includes a plurality of resistors is connected to an amplifier block 73 that includes a plurality of amplifiers by a connector bus 76, and the amplifier block 73 is connected to a capacitor block 74 that includes a plurality of capacitors by a connector bus 77.
The signals received at a receiving terminal 71 connected to the resistor block 72 are filtered by the filter circuit 70, and thereafter is output from an output terminal 75 connected to the amplifier block 73 to the outside of the filter circuit 70.
FIG. 8 is a circuit diagram showing the detailed configuration of the filter circuit 70. The filter circuit 70 is a fourth-order filter circuit, and the amplifier block 73 is provided with four amplifiers 73a. The capacitor block 74 likewise has four capacitors 74a. Each of the capacitors 74a comprises two capacitor elements. The resistor block 72 has four resistors 72a, and each of the resistors 72a comprises a plurality of resistor elements. An amplifier 73a in the amplifier block 73 that is positioned at the rightmost side in the figure is connected to an output terminal 75 of the fourth-order filter circuit 70.
Because this conventional filter circuit 70 has the resistor block 72, the amplifier block 73, and the capacitor block 74 as shown in FIG. 8, it requires a large mounting area. Particularly, the amplifier block 73 and the capacitor block 74, which include a plurality of amplifiers 73a and a plurality of capacitor 74a, respectively, have large areas and therefore necessitate a larger mounting area than the resistor block 72, which includes a plurality of resistors 72a. As a result, the front end having three filter circuits 53 to 55 as shown in FIG. 6 inevitably requires a considerably large circuit scale, which is undesirable.
In addition, it is feared that distortion characteristic of the signals degrades because, for the purpose of switching of the three filter circuits 53 to 55, the input switching circuit 57 and the first and the second output switching circuit 56 and 63 are provided in the signal communication channel.
It is an object of the present invention to provide a filter circuit in which the circuit scale can be reduced and the signal quality improved, and to provide a front end for an asymmetric communication system in which the filter circuit is incorporated, as well as a communication device incorporating the same.
In order to achieve the foregoing and other objects, the invention takes advantage of the fact that, in cases where a plurality of kinds of filter circuits are to be selectively formed, it is possible to configure amplifier blocks having the same value and capacitor blocks having the same value if the resistance values of the resistor blocks in the filter circuits are adjusted, although the values of the amplifier blocks and the values of the capacitor blocks incorporated in the respective filter circuits are different among the amplifier blocks and the capacitor blocks, respectively; and accordingly, in all of the filter circuits, one amplifier block and one capacitor block are commonly used for the amplifier block and the capacitor block that constitute each of the filter circuits, in order to reduce the mounting area.
More specifically, the present invention provides a filter circuit comprising: an amplifier block including a plurality of amplifiers; a capacitor block including a plurality of capacitors, the plurality of capacitors being connected to the plurality of amplifiers included in the amplifier block respectively; a plurality of resistor blocks each including a plurality of resistors; and a resistor block-switching circuit for selecting one of the plurality of resistor blocks and connecting the plurality of resistors included in the selected resistor block to the plurality of amplifiers included in the amplifier block.
Preferably, in the above-described filter circuit, the resistor block-switching circuit is arranged at signal input sides of the plurality of amplifiers included in the amplifier block, the signal input sides being the virtual ground point thereof.
Preferably, in the above-described filter circuit, a first resistor block of the plurality of resistor blocks has a plurality of resistors, the number of which is different from the number of resistors included in a second resistor block of the plurality of resistor blocks; a first filter circuit in which the first resistor block is connected to the amplifier block constitutes a filter circuit having a first ordinal number; and a second filter circuit in which the second resistor block is connected to the amplifier block constitutes a filter circuit having a second ordinal number.
Preferably, in the above-described filter circuit, one of the plurality of amplifiers in the amplifier block that is an output of the first filter circuit is connected to a pair of output terminals of the filter circuit having the first ordinal number; and one of the plurality of amplifiers in the amplifier block that is an output of the second filter circuit is connected to a pair of output terminals of the filter circuit having the second ordinal number.
Preferably, in the above-described filter circuit, an output terminal-switching circuit for selecting one of the pair of output terminals of the filter circuit of the first ordinal number and the pair of output terminals of the filter circuit of the second ordinal number.
In accordance with another aspect, the present invention also provides a front end of a communication system, the front end having: a filter circuit that filters signals, the filter circuit comprising: an amplifier block including a plurality of amplifiers; a capacitor block including a plurality of capacitors, the plurality of capacitors being connected to the plurality of amplifiers included in the amplifier block respectively; a plurality of resistor blocks each including a plurality of resistors; and a resistor block-switching circuit for selecting one of the plurality of resistor blocks and connecting the plurality of resistors included in the selected resistor block to the plurality of amplifiers included in the amplifier block.
Preferably, in the above-described front end of a communication system, the communication system is an asymmetric communication system in which an upstream data rate and a downstream data rate are different from each other.
Preferably, in the above-described front end of a communication system, the asymmetric communication system is an ADSL communication system.
Preferably, in the above-described front end of a communication system, the filter circuit comprises a first filter circuit for received signals and a second filter circuit for transmitted signals; and the first filter circuit and the second filter circuit are not electrically connected by signal wiring and a switch circuit, the first and the second filter circuits being electrically separated from each other.
The present invention also provides a communication device comprising: the foregoing front end; a signal processing circuit for digitally processing a signal received by the front end and a signal transmitted from the front end; and a converter circuit for converting the signal that has been digitally processed by the signal processing circuit into an analog signal and converting an analog signal that is input from a predetermined voice communication terminal to the signal processing circuit into a digital signal.
As described above, the present invention does not prepare a plurality of filter circuits each having an amplifier block, a capacitor block, and a resistor block. Instead, only one amplifier block and one capacitor block are provided while a plurality of resistor blocks are provided, and any one of the plurality of resistor blocks is selected by switching so as to be connected to the amplifier block and the capacitor block. Therefore, only one amplifier block and one capacitor block are sufficient to construct a plurality of filter circuits having differing cutoff frequencies, and thus, the circuit scale is effectively reduced.
Moreover, according to the present invention, because the resistor block-switching circuit is arranged at each signal input side of a plurality of amplifiers, which is the virtual ground point thereof, the existence of the switching circuit does not affect the signal filtering process; therefore, crosstalk or the like can be prevented and occurrences of signal distortion are reduced, whereby the signal characteristic is improved.
Furthermore, according to the present invention, it is possible to change filter circuits having differing filter orders.
In addition, according to the present invention, the first filter circuit for received signals and the second filter circuit for transmitted signals are electrically separated from each other. Therefore, it is not necessary to electrically connect the signal-receiving side and the signal-transmitting side using signal wiring, switch circuits, or the like, and consequently, the layout configuration becomes simple.